One of several mass extinctions that have occurred over time, this one happened at the time boundary between the Triassic and Jurassic periods, 200 million years ago. It wiped out at least half of all species on Earth, including the last of the mammal-like reptiles, leaving mainly dinosaurs.

This enormous loss of species occurred over a period of less than 10,000 yearsalmost instantly, in geological time.

"This is the first time ever that we can see how sudden this event was," said Peter Ward, a University of Washington paleontologist who led the team that reported the new research in the May 11 issue of Science. "It was very quick, not a long, protracted episode."

The findings were based on fossil records of one-celled organisms, called protists, which were found at a remote site in British Columbia.

The fossils showed there was sharp decline in the population of a certain organism. It coincided with scientific data indicating that at about the same time, Earth experienced an abrupt drop in levels of organic carbon (which is one measure of the amount of life on Earth).

Enter Jurassic Park

The rapid disappearance of so many species at once had a major influence on the pattern of evolution, shaping the nature of the life that followed, Ward explained.

"This extinction really opens up the age of dinosaurs. This starts Jurassic Park," he said.

"If we didn't have this mass extinction, the age of dinosaurs as we currently know it now may have been very different," he explained. "The big, herbivore dinosaurs we all think of may never have evolved, because other groups might have emerged if the smaller mammal-like reptiles hadn't been wiped out."

Although there is no definitive evidence yet of what caused the sudden demise of so many species, the suddenness of the event is similar to two other, better known mass extinctions. One occurred 250 million years ago at the end of the Permian period, killing off as much as 90 percent of all species. The other happened 65 million years ago at the end of the Cretaceous period, marking the end of dinosaurs.